San Diego County pension officials are suing a 76-year-old retired sheriff’s captain, trying to recover nearly $500,000 in pension benefits mistakenly overpaid to him over 13 years.

The clerical error that allowed the overpayment was discovered in September as the San Diego County Employees Retirement Association prepared to release data on six-figure pensions as part of a public-records request the agency fought in court for nearly a year.

In a release, the pension agency said it attempted to negotiate with retired Sheriff’s Capt. John Burroughs of Fallbrook for eight months before suing on Friday.

The lawsuit claims the overpayments stretched from October 1999 to September 2011 and seeks $475,000 plus interest.

“When an overpayment is discovered, SDCERA must seek to correct it,” the release said. “Ultimately, overpaid monies are public funds and must be recovered to fund the retirement benefits as SDCERA’s fiduciary duty to its 37,000 members.”

Burroughs’ attorney, Michael Conger, said the agency isn’t taking responsibility for its mistake. Conger said Burroughs is very ill, paid most of the benefits in taxes, has no way of getting them back and didn’t know there was an error.

“SDCERA has never made us a settlement offer,” Conger said. “We offered to go to mediation. They didn’t even respond. The man’s not going to live forever and SDCERA knows it, and the tragedy is this is exacerbating his illness.”

The story of how the overpayments were made stretches back to 1988 and a divorce that ordered a 36 percent share of Burrough’s pension benefit be given to his former spouse after his retirement in March 1993 with 34 and a half years of service.

When Burroughs’ former wife died in July 1997, Burroughs sought restoration of his benefits and two years later reached an agreement with the pension system. The agency paid him a lump sum of $51,563 in back-benefits and restored his full monthly pension payment — and then some.

“We were working with an old mainframe payroll system that was not set up to handle these types of transactions. So this was handled using a benefit deduction,” pension system CEO Brian White said via email. “When the court ordered Burroughs’ full benefit reinstated ... the deduction previously paid to the former spouse was removed AND the amount was added back into the gross, resulting in a double correction and error.”

Conger said SDCERA assured the retiree at the time that his new benefit amount was correct.

“’We’re really sorry, this time we have it right,’” Conger said Burroughs was told. “And now all these years later they are saying, ‘Oops! We made a mistake.’”

According to SDCERA, the error remained undiscovered until September 2011. The agency was pulling information from its old mainframe and verifying its accuracy before fulfilling a records request filed by the California Foundation for Fiscal Responsibility.